EXCLUSIVE: An Auckland-born scientist has admitted in a leaked manuscript to giving his cancer-ravaged mother a lethal overdose of morphine.

Sean Davison published a book last month about his 85-year-old mother's final days, without disclosing the role he played after his mother summoned him home to Dunedin to help her die.

However, a copy of his original manuscript, supplied anonymously to the Herald on Sunday this week, contains the incriminating details that were deleted from the book.

Davison, 47, this week verified the manuscript's authenticity, and said he did not fear police charges.

Asked how he felt about having killed his mother, Patricia Davison, he said: "I have no regrets about what I did. I did the right thing, the compassionate thing."

The married father-of-one had been scheduled to return later this week to South Africa, where he is a professor of biotechnology at the University of the Western Cape. But he has now brought forward his flights.

Since publishing the book and giving his first interview to the Herald on Sunday last month, Davison has begun publicly campaigning for the legalisation of euthanasia.

He has also talked with Lesley Martin, a former nurse from Wanganui who was jailed in 2004 after admitting in a book to giving her cancer-suffering mother an overdose of morphine and smothering her with a pillow. She warned him that he too might be at risk of police charges.

Yesterday, Senior Sergeant Mel Aitken of Dunedin said police would look into the matter: "I've got no further comment to make today."

Davison's memoir, Before We Say Goodbye, was published last month. Adapted from his diary, the book described his mother's repeated requests for him to help her die, and her unsuccessful 33-day hunger strike. But it skirted around the circumstances of her death, in October 2006 at her Dunedin home.

However, in the manuscript he reveals she died hours after he gave her "a lethal drink of crushed morphine tablets" dissolved in a glass of water.

He wrote: "I held it in front of her and said, 'If you drink this you will die.' I really wanted to be so absolutely sure that there was no hesitation.

"She answered, 'You're a wonderful son."'

Learned and strong-minded, Patricia was a retired psychiatrist and GP who had practiced under the name Fergusson. She had colon cancer that spread to her lungs, liver and cerebellum.

The manuscript details the soul-searching and checks Davison made before he killed his mother.

"To kill is to kill, no matter how sweet and right it is. I am committing a premeditated killing," he wrote.

When the Herald on Sunday called Davison, he said: "I didn't intend these details of my mother's death to end up in the public domain, but I realised that was a possible outcome."

He said there had been several copies supplied to friends, family and lawyers.

"One of the reasons this wasn't included in the book was the possibility of legal action, which I think would be absolutely nonsense in this case. Now you've got [the manuscript], it would be intelligent for me to give an intelligent answer rather than people interpreting things on their own."

The book's publisher blocked further comment. Publisher Christine Cole Catley would not discuss the reasons for the cuts. Brendan Malone from Catholic organisation Family Life International said it sounded like mother and son felt they had no other option but assisted suicide which was "a tragedy".

He pointed to other cases of claimed euthanasia where it emerged that the relative had underhand motives. "One of the reasons laws are made to stop this sort of thing is that we just don't know what went on for certain because we only have one side of the story."

Davison was a National Party candidate for Dunedin North in the 1987 election. MPs were allowed to vote according to their conscience on a 2003 private member's bill to legalise physician-assisted suicide. The bill was defeated by three votes.

Lesley Martin believed there were enough parallels between her case and Davison's for the police to consider charges. "He's rolled the dice in the same way I did," she said.

* Extract from Sean Davison's manuscript:
Evening of October 23, 2006

I had already crushed up the morphine tablets and mixed them with water. I told her, "Mum, the time has really come for your wish to come true."

She gave me that disbelieving look of having heard this all too often.

Then she threw me a curve ball, "Bash me on the head!"

I told her that wouldn't kill her.

She replied, "It will help. I can't do anything. I'm finished."

I sat and watched in horror and disbelief. The noise of my mother suffering was agony for me. Yet I wasn't convinced of how genuine her pain was, and it crossed my mind that Mum was acting.

I tried to collect myself. I thought and thought. I knew the answer to the question I was posing myself. I said, "I promise you, Mum, your wish to die will come true tonight."

Her eyes lit up, "What?" It was as if she had finally heard what I had been saying. She now believed what I was going to do.

"Mum, you deserve to die. You don't deserve this suffering."

She quickly agreed. "Yes." She clearly believed that there had been some injustice in the way her life had been prolonged. She then affirmed, "I want to die tonight. I feel dreadful. I feel pain everywhere, and I can hardly talk."

There was no doubt now.

I prepared to give her what I calculated would be a lethal drink of crushed morphine tablets. I held it in front of her and said, "If you drink this you will die." I really wanted to be so absolutely sure that there was no hesitation. She answered, "You're a wonderful son."

I said: "It is not how you planned it. It is not what I planned. This is an event that will live long after you die. Do you want me to help you die?"

"No one will ever know," as if pleading for me to do it. No one need ever know, that was true, but I would. Her destiny was not in her hands, as she'd planned, but in mine.

I held the glass to her lips and gently poured the liquid into her mouth. She had no difficulty in swallowing her last glass. She looked at me with a gentle smile.